Hard disk drives are used as mass storage devices in all manner of computers and have become commonplace. Conventional hard disk drive have a platter that is used to store the data, a spindle that spins the platter rapidly (e.g., 7200 RPM) for reading and writing the data and a motor that rotates the spindle. Additionally, hard disk drives include a read/write head that floats on a cushion of air created as the platter spins underneath the read/write head. The read/write head is coupled to an actuator that places it over the portion of the platter from which data are to be read or to which data are to be written. A disk controller controls the movement of the read/write head, the reading and writing of data and other functions of the hard disk drive.
Hard disk drives cannot be simply turned off, because the cushion of air dissipates before the platter stops spinning. Lacking an air cushion, the read/write head comes into contact with the platter, scoring it and destroying data and perhaps the whole drive.
To prevent this, the disk controller should park the read/write head beside the platter or on a designated landing zone on the platter while there is still a cushion of air. Thus, properly powering-down a hard disk drive involves moving the read/write head to a preferred parking position and then often affirmatively braking the platter. The disk controller requires power to perform these operations. However, that power has been interrupted by virtue of having turned off the hard disk drive.
To provide for a proper power-down, back electromotive force (emf) derived from the mechanical inertia of the decelerating platter is often recovered from wye windings on the motor, rectified in half H-bridge rectifiers and stored in a capacitor to provide temporary power for the disk controller to allow it to perform its power-down operations. Thus, the motor is advantageously used as a generator during a loss of normal operating power to the hard disk drive with the energy stored on the capacitor.
Unfortunately, rectification of the voltage from the motor is not efficient since voltage drops occurs across both upper and lower diodes of the half H-bridges. Since the voltage drop across each diode is about 0.7 volts, a total voltage drop of about 1.4 volts occurs during rectification, resulting in less voltage (and thus power) delivered to the capacitor. As a result, the capacitor is required to be larger, often translating into higher manufacturing costs.
Accordingly, what is needed in the art is a more efficient hard disk drive when recovering energy during a loss of power. Additionally, a method of efficient energy recovery from a spinning platter is also desired.